1. Field of Invention
This invention relates to free radical-curable compositions comprising a plastisol component and a copolymer containing free anhydride functionality and, optionally, other reactive diluents. In particular, this invention relates to curable compositions employing a plastisol, preferably a polyvinyl chloride-("PVC-") containing plastisol, and a low molecular weight crosslinking copolymer component, which copolymer having free anhydride functionality and which is a partial esterification product of a terminally ethylenically unsaturated compound and a polymerizable maleic anhydride.
2. Statement of Related Art
It is generally understood in the art of making thermosetting plastics that plastisols are stable blends of a high molecular weight polymeric resin, typically polyvinyl chloride, in a nonvolatile, nonaqueous plasticizer. The polymeric resin forms the discontinuous phase of the plastisol dispersion or suspension, while the liquid plasticizer forms the continuous phase. Such liquid plasticizers are generally compatible with, but not solvents for, the polymeric resin. The presence of such plasticizers increases the workability and flexibility of the polymeric resin at normal room temperature storage and working temperatures, thereby maintaining the plastisol in a free-flowing state, though of variable viscosity. Plastisols are, therefore, generally considered to be stable dispersions of the two foregoing components, with or without additional volatile diluents and/or other additives. When present, volatile diluents are employed in minor amounts, typically to reduce the viscosity of the plastisol and generally in amounts less than about ten percent (10%) by weight. When volatile diluents are present, such polymeric resin dispersions are sometimes referred to as organosols. Also, when volatile solvents are utilized, they are normally evaporated prior to thermal fusion of the plastisol.
Once formed, a stable plastisol may be heated to form a thermoset polymer, which thermoset polymer may have a variety of physical properties; for example, the thermoset may be tacky, flexible or resilient. It will be appreciated by one skilled in the art that the physical properties of the thermoset product may be controlled through the selection of the resin and plasticizer employed to form the plastisol. Starting with the stable plastisol dispersion, the material may be thickened to a pre-gelled dispersion through the application of moderate heat. At the minimum fluxing temperature, which will vary with the resin and plasticizer that is chosen, the plastisol is converted into a gelled dispersion. Upon the application of additional heat and subsequent cooling, the plastisol forms a thermosetting polymer.
Thermosetting plastisol compositions for coating onto a compressible lamina are disclosed in U.S. Pat. No. 4,388,363 to Fountain. The disclosed plastisol compositions employ a polyvinyl chloride polymer, a polyacrylate (preferably a di- or triacrylate, the latter being a trimethylolpropane triacrylate) crosslinking monomer and a phenolic resin. The thermosetting plastisols of this patent preferably also contain a peroxide free radical initiator, and, optionally, may contain pigments, stabilizers and fillers. Dioctyl phthalate is the preferred plasticizer for the PVC resin.
U.S. Pat. No. 4,613,639 to Blum et al. discloses a process for manufacturing plastisols and organosols using synthetic resins other than polyvinyl chloride; for example, vinyl esters and (meth)acrylate copolymers and/or (meth)acrylate and acrylonitrile copolymers admixed with plasticizers and a protective colloid. Additional synthetic resins having acidic groups, preferably carboxyl groups, on the polymer molecule, include compounds made by the copolymerization of a synthetic resin with acidic monomers, such as olefinically-unsaturated mono- and dicarboxylic acids and the monoesters of the dicarboxylic acids with alcohols of 1 to 20 carbon atoms, including (meth)acrylic acid, maleic acid and half esters of maleic acid with alcohols of 1 to 20 carbon atoms. The acidic groups may optionally be introduced by chemical modification of the synthetic resin. Such modifications may be made to synthetic resins copolymerized with anhydride groups, such as maleic anhydride monomers, and the subsequent reaction of the anhydride groups with water or an alcohol.
U.S. Pat. No. 4,957,955 to Saur et al. describes plastisols manufactured from a dispersion in a suitable plasticizer of (meth)acrylonitrile and styrene or methylstyrene monomers and a carboxylic acid such as (meth)acrylic, itaconic, fumaric or maleic acid and, optionally, in substitution of the carboxylic acid or in addition thereto, an acrylic and/or methacrylic acid alkyl ester of a C.sub.2 to C.sub.12 alcohol, for example, butyl acrylate or 2-ethylhexyl acrylate.
U.S. Pat. No. 4,623,558 to Lin describes certain thermosetting plastisol dispersions comprising a poly(phenylene oxide) powder; a reactive plasticizer including, among other compounds, at least one liquid monomer, oligomer or prepolymer containing at least one ethylenically unsaturated group; and a thermal or photoinitiator, which plastisol dispersions after fluxing can form thermosets after the crosslinking reaction. The liquid monomers, oligomers, and prepolymers are disclosed as including acrylates and acrylate-terminated prepolymers.
U.S. Pat. No. 4,251,618 discloses photoactive, thermally coalescible resin plastisols comprising a polyvinyl chloride polymer having attached to the backbone photopolymerizable or photocrosslinkable groups, which polymer is dispersed in a liquid plasticizer medium also including a photoinitiator, which initiator is either a photoactive substituent present on the backbone of the polymer or a separate component of the plastisol dispersion. Disclosed polymeric compounds include vinyl chloride/acrylic acid and vinyl chloride/maleic anhydride copolymers modified to the half ester with hydroxy-containing cinnamates, benzophenones, and (meth)acrylates (for example, 2-hydroxyethyl(meth)acrylates).